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<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
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<body>
  
</body>
<script>

  //string number boolean null undefined  function 【Object Arrays】

  //null 与 undefined的区别
  //引用类型初始为null
  // let config=null;
  // let config={};
  //基本类型undefined
  // let web=undefined;
  // let web='';

  function star(num=5){
    //函数参数没有默认值，直接打印也是undefined
    // console.log(num);
    for(let i=0;i<num;i++){
      document.write("*");
    }
    return num;
  }

  //函数没有返回值，直接打印是undefined
  console.log(star(100));
  
  


</script>
</html>